This invention relates to a method and an apparatus for imaging a moving object which utilizes the magnetic resonance phenomena (termed "MRI" hereinafter).
The conventional blood vessel imaging techniques are discussed in detail in publication "IEEE, Trans. on Medical Imaging, MI-5", No. 3, (1986), pp. 140-151, for example.
The techniques essentially fall into either the subtraction method or the cancellation method. Both methods use a pulsed magnetic field having a gradient which causes a phase change to occur due to flow in the imaging region. We call this pulsed magnetic field the flow-encode pulse. When the gradient of the flow-encode pulse exists in the flow direction, a phase change dependent on the flow velocity occurs.
The subtraction method implements the subtraction of reproduced images of the flow-sensitive sequence which includes the above-mentioned phase change corresponding to blood flow along the gradient of the flow-encode pulse and the flow-insensitive sequence which has no such phase information. Since the blood flow within a blood vessel forms layers, the imaging based on the phase change information from the flow-sensitive sequence results in the occurrence of phase changes depending on the distance from the center of the blood vessel. Image data cancel each other by being integrated, and therefore no signal is produced from the blood vessel, whereas in the flow-insensitive sequence, the phase is not changed by the movement, and therefore signals are produced from a blood vessel having layers of flow. A static portion produces signals in both sequences. Subtraction of both sequences results in the subsidence of a static portion, and only a blood vessel portion appears as a difference of the two sequence.
The cancellation method reveals blood vessels by a single imaging process. At spin excitation, a radio frequency field equivalent to a 360.degree. pulse is applied and, at the same time, a flow-encode pulse is applied. A static portion restores the original state, producing no signal, whereas a moving portion has its phase changed by the flow-encode pulse, producing a signal. Accordingly, imaging the observation signal reveals only blood vessels.
There is a method of blood flow measurement called "time-of-flight method", although it is not a blood vessel imaging method. A typical practice of the method is that the upstream section of a stream is excited and spins flowing out of it are imaged to produce a picture, as described in publication "Radiology", Vol. 159, pp. 195-199, 1986. The flow velocity can be measured from positions of imaged spins based on the fact that a spin at a high flow velocity is imaged farther from a reference position, and a spin at a low flow velocity is imaged nearer to it.
Among the above-mentioned techniques, the subtraction method and cancellation method make images of all moving parts within the view field, and therefore they intrinsically cannot distinguish the coronary artery running on the exterior wall of the heart from the blood inside the heart which makes it difficult to image the coronary artery.
On the other hand, by application of the time-of-flight method to the imaging of blood vessels, it is possible in principle to image only the coronary artery through the excitation of the upstream section of the coronary artery. For example, U.S. patent application Ser. No. 379,674 discloses an example of blood vessel imaging methods which uses the time-of-flight method. However, because of the movement of the heart to which the coronary artery belongs, it is difficult for the conventional time-of-flight method, in which the position of excitation by a radio-frequency pulse is fixed, to image the coronary artery which changes position continuously.